The multinucleate plasmodium of the myxomycete Physarum polycephalum has been chosen for this study because mitotic events are synchronous in all the nuclei throughout a particular plasmodium, and because vigorous shuttle streaming in endoplasmic channels can be readily observed. Surface cultures of plasmodia will be prepared to study cellular events during mitosis. Using a photometric method involving refraction of a He-Ne laser beam by moving particles in the streaming channel, quantitative measurements will be made of protoplasmic streaming and its decrease or cessation during mitosis. During the period of cessation of streaming, observations with the electron microscope will be made to detect possible changes in the arrangement of actomyosin fibrils in the cytoplasm. The location and amount of calcium accumulation in cytoplasmic vesticles will be measured by a cytochemical method using calcium oxalate precipitation, or by Ca-45 autoradiography in an effort to correlate changes in cytoplasmic calcium location with cessation of protoplasmic streaming. The concentrations of cytoplasmic and nuclear myosin will be measured during mitosis by the quantitative micro-complement fixation technique. The similarity of nuclear and cytoplasmic myosin will be determined by the extent of immunological crossreaction. The location of nuclear myosin with particular emphasis on chromosomal sites will be studied by immunocytochemical methods.